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The Problem Two essential requirements for comfortable and healthy indoor environments are adequate ventilation and good humidity control. Unfortunately in humid climates (which includes the entire eastern half of the U.S.), it is difficult to meet both these requirements without incurring very high utility bills. The fundamental problem is that a conventional cooling coil (whether using chilled water or refrigerant) cannot effectively meet the latent loads from ventilation on very humid days. All conventional chillers and air conditioners dry air by cooling the air below its dewpoint temperature. These systems must run with a wet cooling coil and the air that leaves this coil must be close to saturation. Approximately 25% to 30% of the cooling provided by a conventional DX air conditioner will be latent (i.e., dehumidification). However, in humid climates, the latent load can be between 30% and 60% of the total load, the larger latent loads tending to occur when ventilation is greatest. If a building owner ignores the fact that his air conditioner is not providing sufficient dehumidification, he will create far greater problems. When a building is inadequately dehumidified, people are uncomfortable. A common response is to turn down the thermostat. This creates a cool but clammy indoor environment. And, the high indoor relative humidity that this produces will promote the growth of mold and mildew. The property damage caused by this will cost the building owner more than if he had installed a cooling system that could have prevented the problem.
Several good options exist for meeting high latent loads that retain the basic electric vapor-compression cooling system. In all options, the process air is overcooled to remove extra moisture, but then reheated to maintain comfortable indoor conditions. The method for reheating differentiates these systems. In some, reheating is done using heat from the refrigerant circuit (e.g., heat from the condenser). In others, an air-to-air heat exchanger (either a heat-pipe device, a run-around coil, a heat wheel or a plate-type heat exchanger) is used to move heat from the warm air entering the air conditioner to the cool air leaving. These "high latent" air conditioners are all much more expensive and less efficient than conventional units. The Basic Liquid Desiccant Air Conditioner A liquid-desiccant air conditioner has three main components: the conditioner, the regenerator and the interchange heat exchanger. The conditioner is a parallel-plate liquid-to-air heat exchanger. A coolant, typically cooling tower water (but possibly water from a geothermal well, lake or chilled water loop), flows within the plates and a very-low flow of liquid desiccant flows down the outer surfaces of the plates. Thin wicks on the plate surfaces create uniform desiccant films. The air to be processed flows horizontally through the gaps between the plates. As this humid air comes in contact with the desiccant, water vapor is absorbed. The heat released by this absorption is transferred to the coolant. The air leaves the conditioner drier and at a much lower wet-bulb temperature. The dilute desiccant that leaves the conditioner is pumped to the regenerator. The regenerator has the same configuration as the conditioner: a parallel-plate liquid-to-air heat exchanger. Again, very thin films of desiccant flow in wicks on the outer surfaces of the plates, and air flows in the gaps between the plates. For the regenerator, however, a hot heat transfer fluid flows within the plates. This hot fluid can be supplied by a gas-fired boiler, solar thermal collectors, recovered heat from an engine or fuel cell, or other energy source. As the temperature of the desiccant increases, water evaporates into an air stream that is then discharged outdoors. This regenerator is commonly referred to as a scavenging-air regenerator. The hot, concentrated desiccant that leaves the regenerator and the cool, dilute desiccant that flows to the regenerator exchange thermal energy in the interchange heat exchanger. This exchange increases the efficiency of the regenerator and decreases the cooling load on the conditioner. The efficiency of the regenerator can also be increased by adding an air-to-air heat exchanger to pre-heat the air that enters the regenerator using the warm, humid air that leaves it. (This air-to-air heat exchanger is not shown in the preceding figure.) In both the regenerator and conditioner, the flow rate of desiccant is so low that the falling films on the plates are contained completely within the thin wicks. The air velocity over the films is far too low to entrain desiccant droplets. Since both the desiccant delivery to and collection from the plates are done without creating droplets, desiccant does not carryover and there is no need for droplet filters. The Implementation of Low-Flow Liquid-Desiccant Technology Although many liquids have desiccant properties, solutions of halide salts, particularly lithium chloride and calcium chloride, are the most viable liquid desiccants for HVAC applications. However, the high chloride concentrations in solutions of these salts eliminate even most stainless steels from service in contact with the desiccant. If maintenance is to be acceptable, all wetted surfaces of a LDAC should be a plastic with suitable properties.
A low-flow regenerator functions similarly to a conditioner, the major difference being that now a hot fluid flows within the plates instead of a coolant. The high operating temperatures forces several design changes. As with the conditioner, polymers can best deal with the corrosiveness of the liquid desiccant. Since both the efficiency and water-removal capacity of a scavenging-air regenerator increase with operating temperature, a polymer should be selected that withstands high temperatures, e.g., temperatures on the order of 212 F (100 C). The Benefits of a Liquid-Desiccant Air Conditioner A liquid-desiccant air conditioner can help insure that a building gets adequate ventilation while keeping indoor humidity at healthy and comfortable levels. When applied as a Dedicated Outdoor Air System (DOAS), the liquid-desiccant air conditioner will deliver close to 100% latent cooling. In many applications this deep drying of the ventilation air will handle the entire latent load on the building. Since the central air cooling system now does not need to dehumidify, it can operate with a dry coil at warmer temperatures. As noted earlier, the flow rates of liquid desiccant in both the conditioner and the regenerator are very low--typically a factor of 10 to 20 lower than in conventional packed-bed liquid-desiccant equipment. In addition to eliminating desiccant carryover, low-flow conditioners and regenerators will be more compact, have much lower air-side pressure drops and have much lower pump powers than their high-flow counterparts. The primary energy input to a liquid-desiccant air conditioner is heat. The electrical demand for pumps and fans will be typically less than one-fifth that for a conventional compressor-driven electric air conditioner. The liquid-desiccant air conditioner can provide additional cooling to buildings where electrical service is limited or electrical demand charges are high. The heat needed to regenerate the desiccant can come from many sources. When used with a high efficiency 1½-effect regenerator, which will have a gas-based COP between 1.0 and 1.1, natural gas can be economically used to run the air conditioner. As part of a solar hot-water system or a CHP system, recoverable thermal energy between 160 F and 210 F can be used in a scavenging-air regenerator at COPs between 0.60 and 0.80. |
The
picture at the left shows a plastic-plate heat
exchanger that functions as a 6,000-cfm